add support for multiple texture coordinates
add support for multiple texture coordinates

Written by Alexis Girault

Update the parser to first instantiate the appropriate
number of data arrays needed after figuring the number
of different set of texture coordinates. Each of those
set are associated to different materials (texture),
defined in an .obj file by a line templated as follow:
`usemtl material_name`

In the second parsing of the file, update the current
material name being processed when reaching a line that
start by `usemtl`, then fill in the texture coordinates
accordingly when reaching a line starting by `vt`:
- add the coordinates read at that line to the current material
- add (-1, -1) to the other materials

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